If, Else, If Else Help?

bholtiphone

Hello, I have this script that takes input from three form boxes and inserts it into a mailto href. but when i try to modify it at all it stops working. what i want to do is have if, else, and if else statements that if a user doesnt fill in a box it alerts them what box they didnt fill in and goes back to the input boxes so they can fill it in. ive tried many times to do this but it seems that every time i touch it it messes up